1. Field of the Invention
The present invention relates to a subscriber-line transmission apparatus which is installed between a network-side apparatus and a subscriber-side apparatus.
2. Description of the Related Art
A subscriber-line transmission apparatus is installed between a network-side apparatus, such as an exchange or switch, and a subscriber-side apparatus for the purposes of the prolongation of a communication distance, etc.
In recent years, the exchange has come to include an integrated digital terminal (IDT), and the IDT and a remote digital terminal (RDT) being a subscriber-line transmission apparatus have come to be directly connected by digital circuits or lines such as "DS 1" (digital signal level-1, which shall also be expressed as DS-1 below) circuits.
Heretofore, as illustrated in FIG. 1, the RDT which is connected through the DS-1 circuits with the IDT disposed in the exchange has accommodated subscriber telephones through analog metallic lines. In such a subscriber-line transmission system, as seen from FIG. 1, the RDT is supplied with a master clock through the DS-1 circuits as well as the IDT disposed in the exchange, from a timing source disposed in the exchange, and it is operated in synchronism with the master clock.
Further, the inventors have developed a subscriber-line transmission system called "DS-1 extension system" for the purposes of enhancement in the function of the subscriber-line transmission apparatus, etc.
In the DS-1 extension system, as illustrated in FIG. 2, an RDT is connected through DS-1 circuits, not only with an IDT disposed in an exchange, but also with apparatus called "digital network elements (DNE)" which are network-side apparatus other than the exchange. The RDT is also connected through DS-1 circuits with subscriber-side apparatus (such as customer premises equipment abbreviated to "CPE") other than subscriber telephones, besides accommodating the subscriber telephones through analog metallic lines. The DNE include, for example, a digital cross-connect system (DCS) or a D-4 recommendation channel bank (D4 CHBK) as shown in FIG. 2, or a toll switch not especially shown. On the other hand, the CPE have functions equivalent to those of a switching center of small scale.
In the DS-1 extension system as depicted in FIG. 2, it is required to incorporate the function of transparently connecting the DS-1 circuits associated with the IDT and the DS-1 circuits associated with the CPE, or the DS-1 circuits associated with the DNE and the DS-1 circuits associated with the CPE, respectively. Such a function is called the "un-channelized DS-1 function".
Besides, in the DS-1 extension system as depicted in FIG. 2, it is required to incorporate the function (cross-connect function) of fixedly exchanging channels (time slots) between respective DS-0 (digital signal level-0) connections accommodated in the DS-1 circuits connected to the IDT and correspondent DS-0 connections accommodated in the DS-1 circuits connected to the CPE, or between respective DS-0 connections accommodated in the DS-1 circuits connected to the DNE and correspondent DS-0 connections accommodated in the DS-1 circuits connected to the CPE. Such a function is called the "channelized DS-1 function".
Here in the DS-1 extension system as depicted in FIG. 2, the exchange accommodating the IDT, and the DNE are respectively operated in synchronism with timing sources A and B which are independent of each other. Accordingly, a technique for establishing the synchronization of the operation timings of the exchange accommodating the IDT, the DNE and the CPE becomes very important in the RDT which constitutes the DS-1 extension system.
The RDT constituting the DS-1 extension system needs to execute cross-connect processes for the plurality of DS-0 connections accommodated in the plurality of DS-1 circuits to which the various devices as shown in FIG. 2 are connected. For this purpose, the RDT needs to operate in synchronism with common timings, especially with the operation timing of the exchange accommodating the IDT to which it is connected. In general, accordingly, the RDT executes the cross-connect processes, etc. with a clock (RDT clock #1 shown in FIG. 3) which is extracted from the DS-1 circuit connected to the IDT within the exchange, which is synchronized to the timing source A within the exchange, and which is used as a master clock.
Further, the RDT delivers cross-connected communication data to the DS-1 circuit laid toward CPE, in synchronism with the RDT clock #1. Here, synchronization to an identical clock is required of the communication data on the up DS-1 circuit and communication data on the down DS-1 circuit, the up and down DS-1 circuits being terminated by one device. When such a condition is fulfilled, it is usually said that a loop timing is guaranteed. Accordingly, the CPE connected to the RDT delivers the communication data to the DS-1 circuit laid toward the RDT, in synchronism with a clock extracted from the DS-1 circuit laid from the RDT, in order to guarantee the loop timing for the up and down DS-1 circuits at which this CPE terminates. If the loop timing is perfectly guaranteed in the CPE, the RDT can operate in synchronism with the RDT clock #1 to receive the communication data transferred thereto on the DS-1 circuit laid from the CPE and to execute the cross-connect process for the received data. In fact, however, the loop timing is not always perfectly guaranteed in the CPE, and the RDT clock #1 and a clock on the DS-1 circuit laid from the CPE (a clock #3 from the CPE as shown in FIG. 3) are not always in synchronism with each other. Posed as a problem in such a case is how the RDT establishes the synchronization in order to execute the reception process and the cross-connect process for the communication data transferred thereto on the DS-1 circuit laid from the CPE.
Nevertheless, a technique for such synchronization establishment has not hitherto been known.
Meanwhile, the DNE connected to the RDT delivers communication data to the DS-1 circuit laid toward the RDT in synchronism with a clock (clock #2 from the DNE as shown in FIG. 3) which is generated by the timing source B built in this DNE. On the other hand, the RDT must deliver communication data to the DS-1 circuit laid toward the DNE, in synchronism with a clock extracted from the DS-1 circuit laid from the DNE, in order to guarantee a loop timing for the up and down DS-1 circuits which correspond to the DNE. As stated before, however, the RDT operates in synchronism with the RDT clock #1. It is therefore posed as problem how the RDT establishes the synchronization in order to execute the cross-connect process and the transmission process with the loop timing guaranteed, for the communication data transferred thereto on the DS-1 circuit laid toward the DNE.
Nevertheless, a technique for such synchronization establishment has not hitherto been known, either.
The problems mentioned above are not involved only in the DS-1 extension system, but they are generally existent in relation to a subscriber-line transmission apparatus which terminates a plurality of circuits synchronized with a plurality of timing sources and which executes cross-connect processes among the circuits.